1. Field of the Invention
The present invention relates to a method for analyzing impurities contained within a silicon wafer in a simple manner with high accuracy and sensitivity.
2. Description of the Related Art
It is well known that, when there are present impurities such as metal elements (Al, Na, Fe, Cr, Ni, Cu, etc.) on or within a silicon wafer, these impurities have a harmful effect on electrical characteristics of a resultant semiconductor device. For this reason, it becomes essential to minimize or reduce the quantity of such impurities to a level as small as possible, which requires accurate analysis of the type, surface concentration and content of the impurities.
Conventional methods for directly analyzing metallic impurities on a silicon wafer or within a surface layer thereof include a secondary ion mass spectroscopy and a total reflection X-ray fluorescence analysis. In other matters, samples to be analyzed are taken from the surface layer of a silicon wafer. For example, a native oxide film on the surface layer of the silicon is dissolved with a vapor of hydrofluoric acid (HF), then a resultant dissolved solution is recovered as a sample, and then analyzed.
In both of these analysis methods, however, impurities present merely within the surface layer of the silicon wafer have been analyzed and it has been difficult to analyze impurities existing deep within the silicon wafer. Further, in the case where more impurities are contained deep within the wafer rather than the surface layer thereof or where the diffusion velosity of the impurities is fast, it has been impossible to attain its accurate quantitative evaluation.
Meanwhile, conventional methods for analyzing impurities in the bulk of a wafer include a method for analyzing a solution obtained by dissolving the entire wafer in a chemical agent solution and a secondary ion mass spectroscopy.
However, the former method has had a problem that not only a large quantity of the aforementioned solution is necessary with insufficient analysis accuracy and sensitivity but it also takes a lot of time to perform the analysis and the sample wafer is totally wasted. The latter method, on the other hand, has been disadvantageous in that expensive facilities are required and information obtained through a single sputtering analysis corresponds merely to that part of the sample which is as small as, e.g., 2 mm.sup.2 (actually, a zone of 1 mm in diameter being able to be analyzed), which requires a lot of analysis time. In addition, the latter method is a destructive test.